Aircraft: LIGHT TRANSPORT; LOW WING; 2 TURBOPROP ENG
Phase of flight: Cruise
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance

I had just leveled off at 16;000 [feet] when ATC asked for what my assigned altitude was. I responded 16;000 [feet] [feet] and they informed me that I was showing 17;500 [feet]. I verified that I was showing 16;000 [feet] on my altimeter and then looked at my co-pilot altimeter and realized it was showing 17;600 [feet] and climbing. I communicated this discrepancy to ATC and began to troubleshoot while trying to maintain 16;000 [feet] on my altimeter until I had verified where the discrepancy lay. ATC had me turn off the mode C part of my transponder at this time. My first reaction was to run the icing encounter in flight do list in case it was an issue with icing. I then consulted the QRH (Quick Reference Handbook) and ran the static instrument malfunction do list. This 16;000 [feet] helped by raising my altimeter to approximately 16;700 [feet]. ATC then gave me a descent to 11;000 [feet]. At this time I looked at my airspeed indicator and noticed that while the captain speed was showing 160 knots the co-pilot airspeed was showing approximately 230 knots. I reduced power and referenced the co-pilot airspeed indicator for my speed in the descent. I leveled out at 11;000 [feet] on the 17;600 [feet]pilot altimeter and indicating 12;000 [feet] on the co-pilot altimeter. I requested to turn the mode C on my transponder back on and asked ATC for a readout on my altitude. They advised I was showing 12;000 [feet]. I then advised I was continuing my descent to 11;000 [feet] based off of my co-pilot altimeter. I subsequently flew the aircraft based on the co-pilot instruments. While descending I noticed that the airspeed and altitude on the pilot side was more closely matching to the co-pilot side as I got lower. I was able to visually identify the airport of destination and used the PAPI (Precision Approach Path Indicator) to verify my glide slope to the airport. Upon landing my speeds were within approximately 5 knots of each other and the altimeter was less than 200 foot difference. After shutdown the altimeters are less than 30 feet off of each other.I became aware of the event as I was leveling off at what I thought was 16;000 [feet] ft when ATC queried what my assigned altitude was. I then started to work the problem and work to rectify the instrument mismatch.There was a mismatch between the pilot static systems of my aircraft.I advised ATC that I was having an instrument mismatch and began working the problem to find a solution. I initially thought it might be an icing issue so I run the icing in flight do list. When that failed to resolve the issue I went to the QRH and ran the Static source malfunction do list. When that partially solved the issue I asked ATC to verify altitudes to work to determine what instruments were reading the most accurate information so I could plan and execute a plan of action as safely as possible. Not so much how to keep this from happening again but additional steps I could have taken. Once it became obvious to me that I was having an instrument mismatch and potential malfunction I should have declared an emergency. ATC was extremely helpful and assisted me completely but this could have been much more dangerous if the weather had been worse. I should also have requested the ILS (Instrument Landing System)to the opposite direction runway to have had that as a reference to the landing and terrain clearance instead of just the PAPI. I initially opted not to do this because it would have put me back in the clouds while getting vectored out for it. Air taxi pilot reported pitot static system anomaly.